What is causing my bike to overheat?

  • Hi guys,
    I posted here a couple of months ago when I was having trouble with a leaking carb.

    I tried everything to sort my carb out, but It just wouldn't stop leaking...
    Last week I bit the bullet and just brought a refurbished carb for £180 and the leaking has stopped.

    But now I have an even bigger problem,

    The bike is still running very odd, Its hard to explain exactly.
    It feels very sluggish, The power band feels delayed, like If I pull the throttle back, nothing happens, then it all comes at once. It doesn't feel safe. It feels like something is blocked... I had to ride up a hill in 1st because it has no power.

    Also it's overheating like crazy, I rode it for about 5 minutes to try the new carb and the temp gauge was nearly in the red. (I've never seen it that high).

    Can anyone tell me what might be my problem, Because I'm at a complete loss.

  • @dtr2003 A very Lean fuel condition.

    Check you have the correct size Jets, Your Air fuel mixture has become imbalanced, and is now 'Lean'.

    Important to check for any air leaks around the carburettor inlet and Airbox joins

    It has to be sealed to maintain the correct 'fuel to air mixture', which your bike is exhibiting massive signs of not having!

  • Ok, thanks for pointing me in the right direction.
    I'm not very knowledgable about motorbikes, but I have a friend coming to help me at the weekend.

    I'm not sure what I should do about the jets, because I don't think the exhaust is standard.

    The previous owner must have changed the jets with the exhaust because it had a 240 main jet in, when the manual says to use a 210 main jet.

    Should I buy the original yamaha exhaust so I can get the jetting exactly how the manual says?

    I'm sorry if this is confusing
    Do you mind if I come back at the weekend and ask a few more questions when I have more details?

    Thanks for the help, I really appreciate it.

  • @dtr2003 ask away, main things to check in my opinion is the fixing make sure everything that is anything to do with the carb is “air tight” what main jet is in the new carb? I’d put the one out of the leaking one in it and that should be your jetting just about done and no don’t buy a standard exhaust that’s just not necessary

  • Ok guys, You've been a big help,

    The new carb has a 240 main jet too I think, but I'm not certain.
    Should I put a 210 in?

    I'll take off the carb at the weekend, double-check the jets, float height, and look for any leaks and adjust the screws on the carb.


  • Ask away, but I'd be very surprised if the suggested comments are you problem.

    The bike would run hot if it was lean, but usually it creates death ash, under load and kills the engine.

    In my experience, I would not ride the bike until you've got this problem nailed.

    I would check first to ensure there is no air leaks, and that you do indeed have coolant. A working thermostat and all the fluids are there as expected.

  • also check operation of water pump, when warmed up and thermostat open you will see turbulence with the RAD cap off, take it off prior to warming bike though as pressure could be present if you don't. Put my 88 bike right back to standard with 210 main jet and standard exhaust, Calum is far more knowledgeable than me on the carbs so agree air leak or similar could well be making your bike super lean but need to check what's the setup of your new carb. If you confirm all is well with that rules a lot out. Good luck!

  • Yeah I'm not saying it's not fueling, but if the bike was so lean to cause the coolant to rise, then the affects to your engine are going to be horrendous.

    Usually, when running lean, preignition or detonation occurs. Both of which are engine killers.

    They tend to happen under load when the fueling is lean causing the combustible mix to preignite before the designated ignition point. Hammering the engine internals to pieces.

    It's so bad for the engine that if this were the case, I'd be looking to whip the cylinder off and check the tolerances.

    But as said, these things happen under load, and I know you've tried riding it (DON'T) but seriously the engine would destroy itself pretty quickly if it was that lean.

    So, what are you left with. Well the other thing is the coolant system is naff. Probably more likely, however the bike wouldn't be so bad to run, especially when cold. And it would take a bit of time before it became a noticeable problem.

    So it sounds to me like there are multiple issues here, possibly carburetor related, but does sound coolant related.

    I'll have a think.

  • 210 is the main jet size for later bikes, RE/X models
    240 main jet is the earlier ones I believe, same carb though. Doesn't explain your lean running issue though

  • Main jet only really controls the fuelling at wide open throttle. The needle is going to restrict flow at the other times.

    As pointed out, this isn't strictly true, but it can be used in this example when at no load

  • @calum yeah I know, just pointing out the differences. It's also worth noting that the later carbs have 2 positions for the needle clip whereas the early ones have more.

  • guy's slow down you all made some good points here but its a lot of opinions and the guys, not a DTR fanatic or bike mechanic so calm down personally I wouldn't do anything till I checked the coolant level as there could have been air in the system from changing the carb
    next the plug to see if that shows me its lean
    but if the bike is ticking over ok like it's not fluctuating revs that much but when revved a bit it takes awhile to return
    or it climbs revs when ticking over then ye its most probably lean
    and like @Calum said don't be riding it as you might not have a bike for long if you do

    pictures and videos make it easier for people to see and hear the problem to help you so if you can get some up bud

  • You guys have given me a lot to work with...

    No, I definitely won't be riding it anywhere when it's running like this.

    I'll print off this whole thread and show it to my friend who knows a lot more about bikes than me...


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